.NET Asynchronous Programming | wpf-dev-pack

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.NET Asynchronous Programming | wpf-dev-pack

.NET Asynchronous Programming

A guide for APIs and patterns for efficient asynchronous programming.

Quick Reference: See QUICKREF.md for essential patterns at a glance.

1. Core APIs

API	Purpose
`Task`	Async operation (no return value)
`Task<T>`	Async operation (with return value)
`ValueTask<T>`	Optimization for high-frequency calls
`IAsyncEnumerable<T>`	Async streams

2. Task vs ValueTask

2.1 When to Use Task

Most async operations
When actual async work always occurs

2.2 When to Use ValueTask

When synchronous completion is frequent (cache hits)
High-frequency call methods

// Use ValueTask when cache hits are frequent
public ValueTask<Data> GetDataAsync(string key)
{
    if (_cache.TryGetValue(key, out var cached))
    {
        // Synchronous return (no Heap allocation)
        return new ValueTask<Data>(cached);
    }

    // When async work is needed
    return new ValueTask<Data>(LoadFromDbAsync(key));
}

2.3 ValueTask Cautions

// ❌ Bad example: Awaiting ValueTask multiple times
var task = GetDataAsync("key");
var result1 = await task;
var result2 = await task; // May cause error!

// ✅ Good example: Await only once
var result = await GetDataAsync("key");

3. ConfigureAwait

// Use ConfigureAwait(false) in libraries
public async Task<string> FetchDataAsync()
{
    var response = await _httpClient.GetAsync(url)
        .ConfigureAwait(false);
    return await response.Content.ReadAsStringAsync()
        .ConfigureAwait(false);
}

4. Async Streams (IAsyncEnumerable)

public async IAsyncEnumerable<Data> GetDataStreamAsync(
    [EnumeratorCancellation] CancellationToken ct = default)
{
    await foreach (var item in _source.ReadAllAsync(ct))
    {
        yield return await ProcessAsync(item);
    }
}

// Consuming
await foreach (var data in GetDataStreamAsync(ct))
{
    Console.WriteLine(data);
}

5. Cancellation Token

public async Task<Data> LoadDataAsync(CancellationToken ct = default)
{
    ct.ThrowIfCancellationRequested();
    return await _httpClient.GetFromJsonAsync<Data>(url, ct);
}

// Setting timeout
using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
await LongRunningOperationAsync(cts.Token);

6. Concurrency Control

private readonly SemaphoreSlim _semaphore = new(maxCount: 10);

public async Task ProcessWithThrottlingAsync(Data data)
{
    await _semaphore.WaitAsync();
    try
    {
        await ProcessAsync(data);
    }
    finally
    {
        _semaphore.Release();
    }
}

7. Anti-patterns

// ❌ No async void (cannot handle exceptions)
public async void BadMethod() { }

// ✅ async Task
public async Task GoodMethod() { }

// ❌ No .Result, .Wait() (deadlock risk)
var result = GetDataAsync().Result;

// ✅ Use await
var result = await GetDataAsync();

// ❌ Unnecessary async/await
public async Task<Data> GetDataAsync()
{
    return await _repository.GetAsync();
}

// ✅ Direct return
public Task<Data> GetDataAsync()
{
    return _repository.GetAsync();
}

8. References